Sputter target, method for manufacturing a layer, particularly a tco (transparent conductive oxide) layer, and method for manufacturing a thin layer solar cell

Abstract

In the present invention a sub-stoichiometric ceramic ZnOx:Al target, with 0.3<x<1, is used for depositing a ZnO:Al layer in a reactive sputtering process. The process is carried out in an Ar / O2 atmosphere. The diagram depicts the deposition rate R depending on the oxygen flow in a sputtering process according to the present invention compared with a conventional sputter process using a stoichiometric ZnO target. The upper line x<1 indicates the deposition rate R when using the inventive target and process. The lower line x=1, for comparison only, indicates the deposition rate R when using a stoichiometric ceramic ZnO target. It can be seen from the diagram that both processes are quite stable as there are no steep slopes when varying the oxygen flow. However, the line x<1 is above the line x=1. Therefore, a working point P may be selected which has a higher deposition rate R than a corresponding working point P of a corresponding ceramic target. A higher deposition rate, however, entails a lower bombardment of the deposited layer with oxygen ions. Therefore, the quality of the ZnO:Al layer is improved as far as the conductivity and the etchability of the layer are concerned.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a sputter target for the use in a reactive sputtering process for depositing a layer on a substrate, comprising at least a first metal element Me1 of the periodic table, and a quantity of oxygen. Furthermore, the invention relates to a method for manufacturing a layer on a substrate, particularly a TCO (Transparent Conductive Oxide) layer, including providing an above-mentioned sputter target in a process chamber. The invention also relates to a method for manufacturing a thin layer solar cell, comprising the step of: a) depositing a layer on a substrate using a method mentioned above.DESCRIPTION OF THE PRIOR ART[0002]Sputter coating is a well-known method for depositing thin films of different materials on a substrate. Sputter processes include eroding material from a target which is then deposited on a substrate. Sputtering processes are often used to deposit metal thin films on a substrate in an inert gas atmosphere, e....

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Benefits of technology

[0008]It is an object of the present invention to provide a sputter target and a method for manufacturing a layer, particularly a TCO (transparent conductive oxide) layer, as well as a method for manufacturing a thin layer solar cell, wherein the amount / rate of oxygen deposited in the coating film may be controlled while maintaining a high sputter rate, low oxygen bombardment of the deposited film and a stable coating process.Technical Solution

[0012]According to the invention, in a sputter deposition process a sub-stoichiometric ceramic target, e.g. a ZnOx target with x<1 is provided. The sputter process is performed in a reactive Ar / O2 atmosphere. The inventors have found out that by using a sub-stoichiometric target some advantages of ceramic targets may be combined with advantages of a metallic target. First, the process may be stabilized due to the fact that ceramic targets contain a particular amount of oxygen, i.e. there is an intrinsic amount of oxygen which prevents the targets from changing their characteristics significantly within a small variation of the reactive oxygen flow in the process chamber. Furthermore, the flow / amount of oxygen provided during the process may be varied. This allows varying the deposition rate, particularly to increase the deposition rate, while decreasing the bombardment of the coating film with oxygen ions. Furthermore, the stoichiometry between Zn and O in the deposited film may be adjusted to a predetermined value. By means of the inventive target oxidic layers, e.g. TCO (transparent conductive oxide) layers may be deposited in an improved process.

[0015]Particularly, said sputter target includes an amount of a second metal Me2. In a number of applications, for example when producing thin layer solar cells, a TCO layer doped with a metal, e.g. Al and / or Nb, is required. The amount of Al and / or Nb in the ceramic layer, e.g. ZnO or TiO2, ensures the proper functionality of the coating film. The metal doping increases the conductivity of the deposited TCO layer.

[0018]Due to the fact that a bombardment of the coating film with oxygen ions is considerably reduced during the deposition process, the quality of the n-conductive ZnO:Me layer of a thin layer solar cell is considerably improved. Furthermore, in solar cell applications it is necessary to provide an etching step, particularly by using hydrofluoric acid, to structure the TCO layer. The inventors have found out that when using the sputter target and method according to the invention for manufacturing a thin layer solar cell, the process of etching may be improved. The bombardment of the n-conductive layer with oxygen ions during the deposition process may be controlled to be sufficiently low.

[0033]Said atmosphere provided in step b) may be a reactive Ar / O2 atmosphere. The flow / content of O2 in the atmosphere within the coating chamber may be varied to provide a process having a high deposition rate, high stability, and the coating film comprises a predetermined (e.g. a sub-stoichiometric or stoichiometric) amount / concentration of oxygen.

[0035]Said method may include a further step: b. Etching said layer, especially ZnO:Al. Particularly, etching is performed in HCl for generating a rough surface on top of the layer. Afterwards, an absorber layer (system) of a solar cell may be deposited on top of the roughened surface. Another layer or layer system / layer stack may be deposited on top of the first layer, e.g. an absorber layer and a contact layer. This layer stack may include additional dielectric layers in order to improve the cell efficiency due to an enhanced optical performance.

Problems solved by technology

However, it can not be avoided that reactions of the reactive gas also occur on the surface of sputter target causing “target poisoning”.

Thus in reactive oxidic sputter processes of metallic targets instabilities of the process may be caused.

For example, poisoning of the sputter target reduces the thin film growth rate, thus reducing the use of reactive gas, and thus resulting in even more target poisoning.

A further negative effect of reactive sputtering in an oxidic atmosphere is that the substrate is bombarded with negatively charged oxygen ions generated on the surface of the target, particularly at low deposition rates.

However, the bombardment with oxygen ions deteriorates the etchability of the ZnO:Al layer.

Furthermore, it is difficult to control or vary the oxygen content obtained in the deposited ZnO film.

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